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Autonomic Pharmacology 1

Dr Keith Brain Institute of Clinical Sciences

Janig, 1995

Sympathetic Parasympathetic

Basic Anatomy of the ANS

NAd

Cholinergic Transmission

•  Nerves which use acetylcholine as a neurotransmitter

choline

HO

Acetylcholine

1.  Supply of transmitter precursor 2.  Synthesis of transmitter 3.  Storage of transmitter 4.  Release of transmitter 5.  Inactivation of transmitter 6.  Feedback inhibition of release

An approach to the cholinergic nerves 1. Supply

•  Nerves cannot make enough choline •  Choline has to be taken up from blood •  Choline comes from diet and from liver •  Uptake into nerve endings via a high-affinity

carrier, Na+-dependent process •  Hemicholinium:

–  is competitive inhibitor of the choline carrier – Causes activity-dependent block of cholinergic

transmission, due to depletion of ACh stores •  No clinical use (as its actions are widespread)

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2. Synthesis

•  ChAT occurs in nerve cytoplasm •  Triethylcholine is also a substrate and this gives

acetyltriethylcholine, a ‘false transmitter’ (see later)

•  ChAT inhibitors are not used clinically

Choline + acetylCoA ® acetylcholine + CoA catalysed by choline acetyltransferase (ChAT) vesamicol

Storage of acetylcholine Store is maintained by energy-dependent pump

Inhibition of pump by vesamicol leads to depletion of stores

3. Storage

ACh

ATP ATP ADP

ACh

synaptic vesicle

Vesicular ACh transporter

4. Release Always requires entry of Ca2+ into nerve ending

Occurs by exocytosis: fusion of vesicle membrane with cell membrane

ACh ACh

•  Massive release and depletion of vesicles evoked by: black widow spider venom (α-latrotoxin)

•  Release blocked by botulinum toxin. Used

clinically to treat blepharospasm, salivary drooling, axillary hyperhidrosis, achalasia (oesophageal spasm) and for cosmetic reasons; but it is also a biological warfare agent

Drugs that affect the release of acetylcholine

4. Release

•  Diffusion is not important, unless cholinesterase is inhibited

•  Main mechanism is hydrolysis by tissue acetylcholinesterase Acetylcholine ⟶ acetate + choline

This is non-reversible

5. Inactivation How is ACh removed from the synaptic cleft?

•  Acetylcholinesterase (AChE) present in nerve and muscle cells, red cell membrane

•  AChE is mainly bound to cell membranes or to basement membrane

•  Soluble form of AChE is secreted from neurons into CSF (diagnosis of spina bifida or CSF leak)

•  Very specific for hydrolysis of ACh

Cholinesterases

5. Inactivation

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•  Only work if there is pre-existing tonic release of ACh •  Clinically most important actions are on:

–  skeletal muscle (reverse neuromuscular blockade; diagnosis and treatment of myaesthenia gravis)

–  CNS (treatment of Alzheimer’s disease) –  Eye (treatment of glaucoma; pupillary constriction)

•  Examples: neostigmine, physostigmine •  Many common insecticides (organophosphates) are

cholinesterase inhibitors (eg. Parathion)

Cholinesterase inhibitors

5. Inactivation

•  Cause: autoantibodies to skeletal nicotinic receptors

•  Symptoms: – muscle weakness –  rapid fatigue (eyelids first; ptosis)

•  Diagnosis: Edrophonium (short acting AChEI; “Tensilon test”)

•  Treatment: eg. Neostigmine (medium-duration AChEI)

Clinical Aside: Myaesthenia Gravis

Topical Aside: Allegations of Chemical Weapons use in Syria

(2013)

•  Symptoms reported included “excess saliva, narrow pupils and vomiting” or “vomiting and have breathing problems” or “some appear to have constricted pupils”.

•  “Those injured in the strike itself had responded ‘very quickly’ to treatment with atropine”

•  All of these features are typical of organophosphate ‘nerve agents’, which act by irreversible acetycholinesterase inhibition. Examples: sarin, soman.

Source: http://www.bbc.co.uk/news/world-middle-east-22557347

•  Non-specific cholinesterase (wrongly called pseudocholinesterase)

•  Found in blood plasma •  Hydrolyses many different choline esters •  Metabolises some drugs, e.g. suxamethonium •  Rare genetic forms without activity, so check

before giving suxamethonium (neuromuscular blocker)

Butyrylcholinesterase

5. Inactivation

•  ACh (muscarinic) inhibits release of ACh (enteric) •  ACh (nicotinic) increases the release of ACh (motor) •  ATP, adenosine inhibit release •  Morphine (opioids) inhibit release (autonomic), leads to

constipation •  Noradrenaline (α-) inhibits release (autonomic)

increases release (motor nerves) Consider what happens in heavy exercise

Cholinergic nerves have presynaptic (or prejunctional) receptors

Note possibility of feed-back inhibition of ACh release by ACh itself or by co-released ATP

6. Feedback

•  Act on nicotinic receptors to increase their activity •  Many also have anticholinesterase activity •  Examples:

–  galanthamine (from the snowdrop) –  rivastigmine

Nicotinic Allosteric Potentiating Ligands

Galanthus sp.

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Acetylcholine Release and Modulation Acetylcholine receptors

Acetylcholine (ACh) is the transmitter at the following sites:

1)  motor nerve endings in skeletal muscle 2)  preganglionic autonomic 3)  postganglionic parasympathetic (eg. vagus to the heart) 4)  postganglionic sympathetic (eg symp. vasoconstrictor)

In other words, skeletal muscle, heart and sympathetic ganglion cells must all have chemically-sensitive sites (receptors) for acetylcholine.

Karlin (2002) Nat. Rev. Neurosci. 3:102-114

X-ray Chrystallography

NICOTINIC RECEPTOR ANTAGONISTS

Selectively antagonised by: (1)   Skeletal muscle:

decamethonium, α-bungarotoxin (with an exception)

atracurium, pancuronium, rocuronium

(2) Autonomic ganglia (and CNS):

hexamethonium

Non-specific antagonists: d-tubocurarine

Banded krait (Bungarus fasciatus)

Ganglion blocking drugs

Hexamethonium was used to treat high blood pressure (hypertension). Problems:

(1) very poorly absorbed from gut; have to be given by injection.

(2) not only blocks sympathetic ganglia, but also parasympathetic ganglia, to give unpredictable side-

effects.

Mechanism of action of Hexamethonium

- Blocks the ion channel

- Chief characteristics which distinguish channel block from competitive block are:

1. Use dependent block

2. Block becomes more effective as membrane hyperpolarized

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Nicotinic cholinoceptors Relative potencies series of agonists:

(1) Skeletal muscle: nicotine > carbachol >> DMPP >>> methacholine

muscarine or (2) Autonomic ganglia (and CNS): nicotine, DMPP > carbachol >>> methacholine

muscarine

DMPP = dimethylphenylpiperazinium

Nicotinic Receptor Agonists

Eg. Nicotine

- Commonly cause desensitisation

- Ganglia are no longer targeted

Clinically because of wide-spread effects

Muscarinic Receptors

- G-protein-coupled receptors

- 7 transmembrane segments.

Muscarine

CH3

CH3

CH3

CH3

Muscarinic Agonists Parasympathomimetics

ACh

Carbachol

Methacholine

Bethanecol

Muscarine

Pilocarpine

Oxotremorine All closely mimic effects of parasympathetic nerve stimulation

Effects of Parasympathomimetics

Cardiovascular - decreased heart rate, cardiac output

Smooth muscle - contracts, vascular dilates via endothelium (EDRF = NO)

Exocrine glands - secrete - sweating, lacrimation, salivation, bronchial secretion

Effects of muscarinic agonists

–  increased secretions (eg. salivation!) –  increased gut motility –  slowing of the heart –  bronchoconstriction –  urinary frequency –  constriction of pupil –  vasodilation, via activation of receptors which

are not innervated

NB: Many of these effects are also present with acetylcholinesterase inhibition

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Clinical uses muscarinic agonists

•  Pilocarpine – Topically for glaucoma (through an action on

ciliary muscle and hence increasing aqueous humor drainage)

•  Muscarinic agonists: for atonic bladder (to increase bladder contraction).

Uses of Parasympathomimetics

•  Glaucoma

•  Suppression of atrial tachycardia (rare use)

Muscarinic Antagonists 5 subclasses - 3 are particularly important

Class Location examples

M1 Stomach, salivary glands

M2 Cardiac M3 Smooth muscle

While there are subtype-specific antagonists, they are rarely used. However, the less-specific antagonists are used.

Clinical Uses of Antimuscarinic Drugs

•  Asthma (ipratropium)

•  To treat bradycardia (atropine)

•  To decrease gut motility; decrease secretions

(pirenzapine)

•  During operations: decrease secretions, decrease

AChEI side-effects (atropine)

•  To dilate pupils (tropicamide)

•  Urinary incontinence (oxybutynin)

•  Motion sickness (hyoscine)

Summary

3 main types of receptors for ACh:

(1)   Skeletal nicotinic: α1

(2)   Neuronal nicotinic: α2-7

(3)   Muscarinic: M1-5

Ligand-gated ion channels

G-protein coupled receptors

Beware the partial agonist (or prolonged agonist exposure) blocking the functional response (e.g. nmj depolarization block)